This invention relates generally to window coverings, and more specifically to (i) a slat window blind (hereinafter sometimes referred to merely as a "blind") which can be sized to the width needed to cover a window at the moment of installation by the ultimate user without the use of hand tools; in other words, a do-it-yourself window slat blind, and (ii) a one-piece crash-proof cord lock, which cord lock is well adapted for use in said blind. The invention is particularly adapted for use in connection with a type of blind in which a number of individual horizontally disposed slats are spaced vertically apart from one another along flexible lines, often called ladder cords, and which have a flat or slightly curved cross sectional configuration. Often such slats are approximately one inch in width and frequently the term "mini-blind" has been applied to this type of article. Although the cord lock has particular usefulness in conjunction with such a blind, the cord lock has separate application in other environments.